Charged particle beam apparatuses have many functions in a plurality of industrial fields, including, but not limited to, inspection of semiconductor devices during manufacturing, exposure systems for lithography, detecting devices and testing systems. Thus, there is a high demand for inspecting specimens within the micrometer and nanometer scale.
Micrometer and nanometer scale process control, inspection or structuring, is often done with charged particle beams, e.g. electron beams or ion beams, which are generated and focused in charged particle beam devices, such as Scanning Electron Microscopes (SEM) or Focused Ion Beam (FIB) tools. Charged particle beams offer superior spatial resolution compared to e.g. photon beams, due to their short wavelengths.
A prominent tool for inspections is the Scanning Electron Microscope (SEM), an example of which is shown in FIG. 1. FIG. 1 illustrates as a typical example of a charged particle beam device a SEM 1, which includes a beam tube 20 having an electron beam source 5, e.g. a thermal field emission cathode, to generate a primary electron beam 7, a high voltage beam tube 9 to accelerate the primary electron beam 7 up to an energy controlled by an anode voltage Vanode, a condenser 11 to improve the electron beam shape, a magnetic focusing lens 13 and an electrostatic focusing lens 14 to focus the primary electron beam 7 onto a specimen 3. The SEM 1 of FIG. 1 further includes an in-lens detector 15, e.g. a position sensitive detector, to detect and evaluate the signal of the secondary electrons 17 which are generated by the primary electron beam 7 on the specimen 3.
The magnetic focusing lens 13 of FIG. 1 consists of a coil 24 and a yoke 26 shaped to generate a focusing magnetic field for the primary electron beam 7. The electrostatic focusing lens 14 of FIG. 1 includes the lower-end elements 9a of the high voltage beam tube 9, the cone-like shaped elements 26a, i.e. conical cap, of yoke 26, and apertures 16 at the apices of the respective elements. The focusing electric field is defined by the geometry of the lower-end element 9a, of the conical cap, their apertures 16 and by the voltages V1 and V2 between the specimen 3 and, respectively, the conical cap 26a and the high voltage beam tube 9. If the electric field between the conical cap 26a and specimen 3 is adjusted in such a way that it decelerates the primary electron beam 7, i.e. if a retarding electrical field is employed, the spatial resolution of the probing primary electron beam can be increased when combined with a magnetic focusing field. More details about the combined electrostatic and magnetic focusing lens, and about the SEM of FIG. 1 in general, can be found in “High Precision electron optical system for absolute and CD-measurements on large specimens” by J. Frosien, S. Lanio, H. P. Feuerbaum, Nuclear Instruments and Methods in Physics Research A, 363 (1995), pp. 25-30.
While a SEM uses a focused primary electron beam to image a specimen, a FIB instead uses a focused primary ion beam, typically gallium ions. During scanning of the primary ion beam over the specimen, secondary electrons and ions are generated which may be collected to form an image of the surface of the specimen. The FIB can also be incorporated in a system with both electron and ion beam columns, allowing the same feature to be investigated using either of the beams. If the following description names “electrons” or an “electron beam” in various respects, it thus intended that this may also be applicable for ions, unless otherwise stated.
For detection and classification of topographic defects, like particles, at the surface of specimens, a good topographic contrast is necessary. In scanning beam applications, topographic contrast may be obtained by detection of secondary electrons or ions having different starting angles from the specimen. In scanning beam tools the secondary electrons or ions produced at the specimen are usually collected over a broad range of starting angles for imaging the specimen. When using for instance a low energy SEM utilizing a retarding field objective lens for imaging a specimen, substantially all secondary electrons produced at the surface of the specimen may be attracted inside the objective lens and may therefore be detected.
However, if the topography of the observed feature is not very pronounced, the topographic contrast is weak and the background signal plays an important role. In view of the above, there is a need for a scanning beam apparatus with improved contrast.